Spread spectrum techniques can be used to provide a degree of covertness and anti-jam capability to a communications system. In transmission, a spreading code is used to expand the bandwidth of transmitted signals. The transmitted signals can have a relatively large bandwidth as compared to the bandwidth of information encoded into the transmitted signals. For intended receivers, the spreading codes are known, and can be removed once synchronization has been obtained. The process of removing the spreading code in the receiver can also provide benefits in reducing the effects of interference and/or jamming (this benefit is sometimes referred to as processing gain). For adversaries who lack knowledge of the spreading code, detection of and synchronization to the transmitted signal can be difficult. Jamming a spread spectrum signal can also be difficult, since, without knowledge of the spreading code, the intended receiver gains an advantage over the would-be jammer approximately equal to the processing gain.
Spread spectrum processing adds some complexity and cost to a communications system. Detection and demodulation of a spread spectrum signal generally includes creating a local replica of the spreading code, synchronizing the timing of the local code replica to the transmitter, and removing the spreading code from the received signal. This additional processing can require additional signal processing hardware to be included in the communications system as compared to a conventional non-spread system.
Operating rates in a spread spectrum system are also typically much higher than conventional (non-spread) systems. For example, a conventional (non-spread) communications system operating at a 1 Mb/s data rate may use digital circuitry clocked at a 2 MHz rate. In contrast, a spread spectrum system operating at the same 1 Mb/s data rate may use a 100 MHz chip rate for the spreading code, and thus operate some of the digital circuitry at 50 or 100 times the clock rate of the conventional (non-spread) system. These higher clock rates can translate into higher hardware cost and higher power consumption.
Increasingly, communications systems are accommodating heterogeneous types of communications terminals. For example, some terminals in a system may be disadvantaged in terms of power availability or other characteristics. To provide communications in a spread spectrum system, it is generally necessary for all of the terminals in communications to operate using the same spreading codes and spreading (chipping) rates. Accordingly, to provide communications in a heterogeneous network has generally involved designing all of the terminals to operate using a particular combination of spreading code types, spreading rates, data rates, and other communications parameters. Such an approach can undesirability limit the overall capability of the communications system. For example, limiting all of the terminals in the network to provide only the minimum performance provided by the least capable terminal in the network may waste higher potential throughput available between some pairs of terminals.